The narrow emission lines of Seyfert 2 galaxies cover a large range of
ionization.
Lines from [OI] to [FeXI] are observed, and the spectrum is
similar to the
narrow line spectrum of Seyfert 1s. The equivalent width of the lines,
relative
to the nonstellar continuum, is systematically larger than in Seyfert
1s. The line
width distribution is broad, from 250 to 900 km s-1,
with an average around
400 km s-1. Blue asymmetry is observed in most lines,
being stronger in lines
of higher critical density. This again is similar to the observations of
Seyfert 1
galaxies, and suggests that the higher density clouds move faster with
respect to the central source.

The nonstellar optical, ultraviolet and X-ray continua of Seyfert 2
nuclei are
very weak but their ratios are not too different from those in
Seyfert 1s, thus
the overall continuum shape is similar in the two sub-groups. The
extrapolation
of the observed ultraviolet continuum to high energies, does not seem to
give
enough ionizing flux to explain the observed emission lines (i.e. the
required
covering factor is larger than 1). Thus, there are several indications
that much of the nonstellar continuum is not observed by us.

The key to the understanding of the difference between Seyfert 1 and Seyfert
2 galaxies is in recent polarization measurements of these objects. The
degree
of polarization is small, 1-3%, but when the polarized flux is
plotted separately
from the rest, as in Fig. 36, it shows, very
clearly, a typical Seyfert 1 spectrum, with strong broad emission lines
of hydrogen and FeII. The small fraction of
polarized flux explains why this detection escaped the notice for many
years.
Currently there is a handful of Seyfert 2 galaxies showing this phenomenon.

The following physical model for Seyfert 2 galaxies has emerged
(Fig. 37). A
"normal" Seyfert 1 nucleus, surrounded by its BLR, is situated at the
center of
the system. A thick torus, of inner radius ~ 1pc and similar
thickness, is present
too. Most of the torus material is in molecular clouds, that are
shielded from
the central radiation by dust and by free electrons that evaporate from the
clouds. The central BLR is obscured from some observers by the molecular
torus. Such observers can only see the small fraction of BLR light that is
scattered in their direction. The intensity of the scattered light
depends on
the Compton depth of the medium, and the degree of polarization on the
observer's viewing angle. Some viewing angles are not obscured, and a
Seyfert
1 type spectrum is observed. The NLR size greatly exceeds the dimension of
the torus, and the narrow emission lines are seen by all observers. However,
the NLR illumination and ionization is not isotropic, because of the torus,
and the observed NLR is likely to attain a jet-like
structure. There are radio
observations and narrow emission line maps that support the claim for a
jet-like NLR in Seyfert 2 galaxies.

Figure 37. A unified model for Seyfert
galaxies. The central source and the BLR are surrounded
by a thick torus of molecular gas. The Seyfert 1 galaxies are those
objects observed from the
pole direction. Seyfert 2 galaxies are those sources whose inner parts
can only be seen through reflected radiation.

The above model provides a natural explanation for the difference between
Seyfert 1 and Seyfert 2 galaxies. One suggestion is that the different
viewing
direction is the only distinction between the two groups of objects. In
this case
the thickness and dimension of the torus can be estimated from the relative
number of Seyfert 1 and Seyfert 2 galaxies. There are difficulties too,
such as
several well studied Seyfert 2s where the polarized flux is extremely
small and
no broad lines are seen. The role of dust is not very clear and heavy
reddening
is likely to be present, at least in some directions. Light scattered by
dust is
polarized in a wavelength dependent way, and there are observational ways to
test this idea. The Compton depth should be large enough to scatter part of
the broad line radiation, but not too large to smear out the line and
continuum variability in Seyfert 1 galaxies. There are theoretical
uncertainties too, to do with the structure and stability of the torus.

An alternative explanation to the differences between the two groups of
galaxies is related to their variability. The broad emission lines of
some Seyfert
1 galaxies exhibit a large amplitude variations. In some objects the
variability
so large that the spectrum at minimum light resembles a Seyfert 2 spectrum.
It has been suggested that some Seyfert nuclei spend a large fraction of
their
active phase in a "turned-off" state, when the ionized flux is too weak
to excite
the gas. The recombination time of the broad line gas is short and the lines
disappear several hours after the continuum decline. The recombination
time of
the low density gas is long enough to show strong narrow lines many
years after
the decline. Such an object, with strong narrow lines and weak
continuum, will
be classified as a Seyfert 2 galaxy. A possible cause for the drop in
luminosity
is a large decrease in the accretion rate. The big continuum bump
observed in
broad line AGNs has been attributed to emission from accretion disks. If
this is
indeed the case then the time scale for its fading is very long. It is
hoped that
future HST observations will help to decide whether the continuum of Seyfert
2 galaxies shows any sign of such a bump.

To summarize, Seyfert 2 galaxies may be Seyfert 1 nuclei that are hidden
in space or in time. This may explain many, perhaps most observed properties
of Seyfert 2s. However, we should not neglect the possibility of the
presence of genuine narrow line objects, with no BLR at all.